digest: do not depend on ext/openssl/deprecation.rb

ext/openssl/deprecation.rb has been removed in ext/openssl upstream[1].

It was originally introduced to detect and prevent linking against
Apple's bundled (and very outdated) version of OpenSSL. It is long gone
and the header files do not exist in current versions of macOS.

The dependency in ext/digest was introduced by commit 26e258c807.
Note that the original issue[2] is resolved without this dependency.
The commit also added the pkg_config("openss") call to match what
ext/openssl does.

[1] https://github.com/ruby/openssl/pull/333
[2] https://bugs.ruby-lang.org/issues/6379

ext/digest/digest_conf.rb

@@ -65,13 +65,12 @@ def digest_conf(name, hdr = name, funcs = nil, types = nil)
 
     dir_config("openssl")
     pkg_config("openssl")
-    require File.expand_path('../../openssl/deprecation', __FILE__)
     if find_openssl_library
       funcs ||= name.upcase
       funcs = Array(funcs)
       types ||= funcs
       hdr = "openssl/#{hdr}.h"
-      if funcs.all? {|func| OpenSSL.check_func("#{func}_Transform", hdr)} &&
+      if funcs.all? {|func| have_func("#{func}_Transform", hdr)} &&
          types.all? {|type| have_type("#{type}_CTX", hdr)}
         $defs << "-D#{name.upcase}_USE_OPENSSL"
         $headers << "#{name}ossl.h"